Heat treatment of nickel chromiumcobalt alloys



United States Patent 3,145,124 HEAT TREATMENT 0F NKCKELCHROMIUM- CQBALT ALLOYS Harold William George Hignett, Hereford, Ernest James Bradbury, Solihull, Ronald Alfred Smith, West Hagley, and David Marshall Ward, Birmingham, England, assignors to The international Nickel Company, Inc, New York, N.Y., a corporation of Delaware No Drawing. Filed Mar. 13, 1962, Ser. No. 179,475

Claims priority, application Great Britain Feb. 17, 1961 6 Claims. (Cl. 140--162) The present invention relates to the heat treatment of nickel-chromium alloys and, more particualrly, to nickelchromium-cobalt alloys of the kind that contain titanium or aluminum or both and can be age hardened.

Heat treatments hitherto used have been of two main kinds, both designed to give the highest possible resistance to rupture under conditions of prolonged tensile stress at high temperature. With this object in view, one widely used treatment described, for example, in UK. specification No. 583,845, consists in heating the alloy at such a temperature and for such a time that a substan tially homogeneous solid solution is produced (so-called solution heat treatment) followed by aging at a temperature low enough to ensure that a coherent precipitate of an Ni (Ti, Al) phase is formed in an extremely finelydivided state. Generally speaking, this requires the use of an aging temperature in the range 600 C.850 C.

The other type of heat treatment is applied to carboncontaining alloys and was based on the discovery that the controlled precipitation of carbides at the grain boundaries in the alloy is of importance in ensuring the highest resistance to rupture. In this treatment an interme diate heating step during which there is no substantial precipitation of a hardening phase is interposed between the solution heat treatment and aging steps and is carried out at a temperature between those used in these latter steps. During the intermediate heating, carbides are precipitated at the grain boundaries and substantially the whole of the precipitation of the coherent Ni (Ti, Al) phase takes place during the aging step which, as the two-stage treatment, takes place in the temperature range from 600 C. or 650 C. to 850 C. Three-stage heat treatments of this kind are described in UK. specifications Nos. 715,140 and 731,441 and are generally satisfactory when applied to alloys having a total content of less than 6% of titanium and aluminum.

It has been found that if alloys, with which the present invention is concerned, are solution heat treated and then heated at a temperature chosen to precipitate carbide at the grain boundaries, the alloys have low impact strength at high temperatures, e.g., 900 C. The high temperature impact strength is of considerable importance when the alloys are used for making rotor blades for gas turbine engines, where there is a danger of the blade coming into contact with the turbine casing or with foreign bodies that may pass through the engine. Improvement in this impact strength is therefore advantageous. Although attempts were made to provide commercially acceptable heat treatments which would induce good high temperature impact strength in alloys having high total contents of titanium plus aluminum, none, as far as we are aware, was entirely successful when carried into practice commercially on an industrial scale.

It has now been discovered that by a specially controlled heat treating process, good high temperature impact characteristics can be developed in alloys rich in total titanium plus aluminum and at the same time good resistance to prolonged stress at elevated temperatures can be maintained.

It is an object of the present invention to provide a novel heat-treatment process for nickel-chromium-cobalt alloys rich in total aluminum plus titanium.

Another object of the invention is to provide novel heat treated nickel-chromium-cobalt alloys.

Other objects and advantages will become apparent from the following description.

The present invention is concerned with alloys having a total titanium and aluminum content of more than 8% and up to 9.5% and which contain about 14.2% to about 15.8% chromium, about 14% to about 25% cobalt, about 3% to about 5.5% molybdenum, about 3% to about 4.6% titanium, about 4% to about 5.4% alumi num, about 0.0 1% to about 0.2% carbon, about 0.01% to about 0.2% zirconium, and about 0.003% to about 0.1% boron, the balance, apart from impurities, being nickel. The amounts of silicon, manganese and iron present as impurities should be as low as possible, the silicon and manganese contents not exceeding about 0.5% and the iron content not exceeding about 1%.

The present invention is based on the discovery that if solution heat treatment is followed by heating at a temperature within a narrow and critical range immediately below the solubility temperature of the Ni (Ti, Al) phase, substantial amounts of this phase are precipitated at the grain boundaries and the impact strength is substantially improved. It is believed that the Ni (Ti, Al) precipitate thus formed contains a substantial amount of the carbon content of the alloy in solution and thus prevents it from separating in a harmful form. Further amounts of the Ni (Ti, Al) phase are precipitated in a coherent form in the body of the grains on cooling to lower temperatures, but at such a rate that in these alloys prolonged aging in the temperature range 600 C. to 850 C. serves no useful purpose.

According to the invention, an alloy of the composition set forth above is subjected to a solution heat treatment comprising heating at a temperature above the solubility temperature of the Ni (Ti, Al) phase but below the solidus temperature of the alloy for at least /4. hour and is then isothermally heated for at least a further A hour at a temperature below, but not more than 50 C. (i.e., 50 in centigrate units) below the solubility temperature of the Ni (Ti, Al) phase and not less than 1075 C.

At temperatures very close to the solubility temperature, the rate of separation of the Ni;,(Ti, Al) phase is inconveniently slow and the isothermal heating temperature is, therefore, advantageously at least 10 C. or even 25 C. below the solubility temperature.

The solubility temperature of the Nl3 (Ti, Al) phase in alloys of the kind with which the present invention is concerned, is determined by heating specimens of an alloy at a temperature and for a sufficient time to ensure that all precipitable phases are in solution, water quench, and then reheat a series of specimens at different temperatures for four hours, again Water quench, and examine them under the microscope for signs of precipitation. The lowest reheating temperature at which no visible precipitation occurs is the solubility temperature.

For alloys having compositions within the range 14.2% to 15.8% chromium, 14% to 16% cobalt, 3% to 4.5% molybdenum, 3% to 4.1% titanium, 4% to 5.1% aluminum, 0.01% to 0.2% carbon, 0.02% to 0.1% zirconium,

0.003% to 0.1% boron, the balance, apart from impur ities, being nickel and the toal content of titanium and aluminum being more than 8% but not more than 9.2%, the isothermal heating temperature is advantageously from 1100 C. to 1125" C.

The solution heat-treatment temperature must, of course, not be higher than the solidus temperature of the alloy to avoid melting and, in practice, the temperature and duration of the solution heating will usually be limited by the need to avoid excessive grain growth. Due regard should also be had to the condition of the material, for example, as to whether it has previously been hot or cold Worked during fabrication. Preferably, the duration of the solution heating does not exceed 4 hours. The duration of the isothermal or substantially isothermal (e.g., plus or minus 5 C.) heating is limited essentially by practical considerations and will usually not exceed 24 hours. A suitable duration is about 1 to about 10 hours.

The impact resistance of the heat-treated alloys tends to fall when the alloys are subjected to severe service heating such as occurs on take-off in an aircraft engine. For test purposes these severe conditions may be simulated by heating the alloy for 16 hours at 980 C. We find that this tendency can be reduced if when the alloys are at the solution-heating temperature they are not allowed to cool below the permissible range of isothermal heating until that heating is complete. Accordingly, direct and rapid transfer of the alloy from a furnace in which the solution heat treatment is carried out to another at the isothermal heating temperature is desirable. In practice, such rapid transfer is often so inconvenient that air cooling, which may be to room temperature, may be preferred.

By way of example, notched impact specimens of an alloy (alloy A) having the nominal composition: Carbon 0.15%, silicon 0.25%, iron 0.4%, chromium 15%, titanium 3.9%, aluminum 4.7%, cobalt 15%, molybdenum 3.5%, boron 0.01%, zirconium 0.05%, nickel balance, which had a solubility temperature of 1140 C. were subjected to different heat treatments and their impact resistance then determined at 900 C. The results are shown in Table I. The impact strengths in the third column are those determined immediately after air cooling from the isothermal heating temperature, while the values in the fourth column were determined after air cooling and reheating at 980 C. for 16 hours.

Table 1 Impact Strength (ft-11).), at 900 0.

Treatment No. Heat Treatment As heat After 16 Treated hrsJ 980 l 1.5 hrs./1,190 C AC; 6 l1rs./1,050

2 2 2 1.5 hrs./l,190 AC; 6 hrs./l,075 8 4 2 2 3 1.5 hrs./l,190 0.; AC; 6 l1rs./l,100

0.; A 16.8 4.4 4 1.5 hrs/1,190 0.; AC; 6 hrs./1,125

0.; 16.8 3.7 1.5 hrs./1,190 C.;DI to 6hrs./1,100

15.4 6.5 6 1.5hrs./l,190C DT to 6hrs ,125

AC=air cooled; DT=direct transfer.

After treatments Nos. 3 to 6 according to the invention, the specimens had satisfactory impact resistance. On the other hand, in treatments Nos.1 and 2 the isothermal heating temperatures were too low and the specimens had much poorer impact properties.

Comparison of treatments Nos. 3 and 4 with Nos. 5 and 6 shows that direct transfer of the specimens from the solution heat-treatment furnace to the furnace at the isothermal heating temperature reduced the extent to which the impact properties fell off after heating equivalent to severe service heating.

According to another feature of the invention, further improvement may be obtained in this respect by cooling the alloy from the isothermal heating temperature to a temperature in the range 950 0-1050 C. and maintaining it at that temperature for a further period. of at least hour. The temperature of this second isothermal heating should be at least as high as the intended service temperature and is preferably 1025 C. The temperature of the alloy must not be allowed to fail below the permissible range for the second isothermal heating until this heating is complete and again it is preferred to transfer the alloy rapidly between the furnaces used for the two isothermal heating steps.

By way of example, notched impact specimens of alloys A and B, of which alloy A had the composition set forth above and alloy B had the composition: carbon 0.14%, silicon 0.15%, iron 0.24%, chromium 14.6%, titanium 3.9%, aluminum 5%, cobalt 14.6%, molybdenum 3.5%, boron 0.016%, zirconium 0.035 nickel balance (solubility temperature 1140 C.), were subjected to diiferent heat treatments and their impact properties at 900 C. were determined. The results are set forth in Table II:

Table 11 Impact Strength (ft. 110.), at 900 Treatment No.

Alloy Heat Treatment As heattreated After 16 hrs. at 980 C.

to 0.5 hr./1,050

, 1s 9 DT to 0.511r./1,025

DT=direct transfer; AC =alr-co0lcd.

Comparison of the results of treatments Nos. 7 and 8 with those of treatment No. 6 in Table I shows the improvement in impact resistance after severe heating that results from a second isothermal or substantially isothermal heat treatment and a similar improvement is shown by comparison of treatments Nos. 10, 11, 13 and 14 with No. 9. Treatments Nos. 11 and 13-16 show how the impact strength after severe heating falls of progressively as the temperature of the second isothermal heating decreases, treatments Nos. 15 and 16 showing no improvement over No. 9. The results of treatment No. 12 show that when the alloy was air cooled before the first isothermal heating a second isothermal heating brought about no appreciable improvement.

The present invention is particularly applicable to the production of gas turbine structures such as turbine blades subjected in normal use to extreme conditions of high temperature, high stress and corrosion and likely to be subjected to impacts while in use at elevated temperatures.

The present application is a continuation-in-part of copending U.S. patent application Serial No. 169,623, filed January 29, 1962, and now abandoned.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be Within the purview and scope of the invention and appended claims.

We claim:

1. A heat treatment process for attaining improved impact strength characteristics together with good stressrupture properties at elevated temperature in nickelchromiunncobalt base alloys containing a total of at least 8% of aluminum plus titanium which comprises heating an alloy containing about 14.2% to about 15.8% chromium, about 14% to about 25% cobalt, about 3% to about 5.5% molybdenum, about 3% to about 4.6% titanium, about 4% to about 5.4% aluminum, the total of said aluminum plus said titanium being more than 8% and up to 9.5%, about 0.01% to about 0.2% carbon, about 0.01% to about 0.2% zirconium, about 0.003% to about 0.1% boron with the balance being essentially nickel at a temperature above the solubility temperature of the Ni (Ti, Al) phase but below the solidus temperature of the alloy for at least 0.25 hour and thereafter precipitating the phase Nig (Ti, Al) without the occurrence of detrimental carbide precipitation at the grain boundaries by substantially isothermally heating said alloy for at least 0.25 hour at a temperature below the solubility temperature of the Ni (Ti, Al) phase and in excess of both 1075 C. and the temperature 50 C. below said solubility temperature of the Nlg (Ti, Al) phase.

2. A heat treatment process as set forth in claim 1 wherein the alloy is heated at a temperature above the solubility temperature of the Ni (Ti, Al) phase for about 0.25 to about 4 hours and is heated substantially isothermally for about 1 to about 10 hours at a temperature of at least 10 C. below the solubility temperature of the Ni (Ti, Al) phase and in excess or" both 1075 C. and the temperature 50 C. below said solubility temperature of the Ni fli, Al) phase.

3. An alloy containing about 14.2% to about 15.8% chromium, about 14% to about 25% cobalt, about 3% to about 5.5% molybdenum, about 3% to about 4.6% titanium, about 4% to about 5.4% aluminum, the total of said aluminum plus said titanium being more than 8% and up to 9.5%, about 0.01% to about 0.2% carbon, about 0.01% to about 0.2% zirconium, about 0.003% to about 0.1% boron with the balance being essentially nickel as heat treated by the process of claim 1.

4. A heat treatment process for attaining improved impact strength characteristics together with good stressrupture properties at elevated temperature in nickelchrornium-cobalt base alloys containing a total of at least 8% of aluminum plus titanium which comprises heating an alloy containing 14.2% to 15.8% chromium, 14% to 16% cobalt, 3% to 4.5% molybdenum, 3% to 4.1% titanium, 4% to 5.1% aluminum, the total of said aluminum plus said titanium being more than 8% but not more than 9.2%, 0.01% to 0.2% carbon, 0.02% to 0.1% zirconium, and 0.003% to 0.1% boron, the balance being essentially nickel at a temperature above the solubility temperature of the Ni, (Ti, Al) phase but below the solidus temperature of the alloy for at least 0.25 hour and thereafter precipitating the phase Ni (Ti, A1) without the occurrence of detrimental carbide precipitation at the grain boundaries by substantially isothennally heating 6 said alloy for at least 0.25 hour at a temperature in the range of 1100 C. to 1125 C.

5. A heat treatment process for attaining improved impact strength characteristics together with good stressrupture properties at elevated temperature in nickelchromium-cobalt base alloys containing a total of at least 8% of aluminum plus titanium which comprises heating an alloy containing about 14.2% to about 15.8% chromium, about 14% to about 25% cobalt, about 3% to about 5.5% molybdenum, about 3% to about 4.6% titanium, about 4% to about 5.4% aluminum, the total of said aluminum plus said titanium being more than 8% and up to 9.5%, about 0.01% to about 0.2% carbon, about 0.01% to about 0.2% zirconium, about 0.003% to about 0.1% boron with the balance being essentially nickel at a temperature above the solubility temperature of the Ni (Ti, Al) phase but below the solidus temperature of the alloy for at least 0.25 hour, precipitating the phase Ni (Ti, Al) without the occurrence of detrimental carbide precipitation at the grain boundaries by substantially isothermally heating said alloy for at least 0.25 hour at a temperature below the solubility temperature of the Ni fli, Al) phase and in excess of both 1075 C. and the temperature 50 C. below said solubility temperature of the Ni ("l"i, Al) phase and thereafter cooling said alloy to a temperature in the range of 950 C. to 1050 C. and maintaining said alloy at that temperature for at least 0.25 hour.

6. A heat treatment process for attaining improved impact strength characteristics together with good stressrupture properties at elevated temperature in nickelchromium-cobalt base alloys containing a total of at least 8% of aluminum plus titanium which comprises heating an alloy containing 14.2% to 15.8% chromium, 14% to 16% cobalt, 3% to 4.5% molybdenum, 3% to 4.1% titanium, 4% to 5.1% aluminum, the total of said aluminum plus said titanium being more than 8% but not more than 9.2%, 0.01% to 0.2% carbon, 0.02% to 0.1% zirconium, and 0.003% to 0.1% boron, the balance being essentially nickel, at a temperature above the solubility temperature of the Ni (Ti, Al) phase but below the solidus temperature of the alloy for at least 0.25 hour, precipitating the phase Ni (Ti, Al) without the occurrence of detrimental carbide precipitation at the grain boundaries by substantially isothermally heating said alloy for at least 0.25 hour at a temperature in the range of 1100 C. to 1125 C. and thereafter cooling said alloy to a temperature in the range of 950 C. to 1050 C. and maintaining said alloy at that temperature for at least 0.25 hour.

References Cited in the file of this patent UNITED STATES PATENTS 2,712,498 Gresham et al July 5, 1955 2,766,155 Betteridge et al. Oct. 9, 1956 2,766,156 Betteridge et al. Oct. 9, 1956 2,977,223 Brown Mar. 28, 1961 

1. A HEAT TREATMENT PROCESS FOR ATTAINING IMPROVED IMPACT STRENGTH CHARACTERISTICS TOGETHER WITH GOOD STRESSRUPTURE PROPERTIES AT ELEVATED TEMPERATURE IN NICKELCHROMIUM-COBALT BASE ALLOYS CONTAINING A TOTAL OF AT LEAST 8% OF ALUMINUM PLUS TITANIUM WHICH COMPRISES HEATING AN ALLOY CONTAINING ABOUT 14.2% TO ABOUT 15.8% CHROMIUM, ABOUT 14% TO ABOUT 25% COBALT, ABOUT 3% TO ABOUT 5.5% MOLYBDENUM, ABOUT 3% TO ABOUT 4.6% TITANIUM, ABOUT 4% TO ABOUT 5.4% ALUMINUM, THE TOTAL OF SAID ALUMINUM PLUS SAID TITANIUM BEING MORE THAN 8% AND UP TO 9.5%, ABOUT 0.01% TO ABOUT 0.2% CARBON, ABOUT 0.01% TO ABOUT 0.2% ZIRCONIUM, ABOUT 0.003% TO ABOUT 0.1% BORON WITH THE BALANCE BEING ESSENTIALLY NICKEL AT A TEMPERATURE ABOVE THE SOLUBILITY TEMPERATURE OF THE NI3(TI, AL) PHASE BUT BELOW THE SOLIDUS TEMPERATURE OF THE ALLOY FOR AT LEAST 0.25 HOUR AND THEREAFTER PRECIPITATING THE PHASE NI3(TI, AL) WITHOUT THE OCCURRENCE OF DETRIMENTAL CARBIDE PRECIPITATION AT THE GRAIN BOUNDARIES BY SUBSTANTIALLY ISOTHERMALLY HEATING SAID ALLOY FOR AT LEAST 0.25 HOUR AT A TEMPERATURE BELOW THE SOLUBILITY TEMPERATURE OF THE NI3(TI, AL) PHASE AND IN EXCESS OF BOTH 1075*C. AND THE TEMPERATURE 50*C. BELOW SAID SOLUBILITY TEMPERATURE OF THE NI3(TI, AL) PHASE. 